A thin film transistor (hereinafter also referred to as “TFT”) as a switching element, for example, is provided on a thin film transistor substrate for each pixel which is a minimum unit of an image.
Further, in general, a thin film transistor using a semiconductor layer made of amorphous silicon is used in a thin film transistor substrate, as a switching element of each pixel which is a minimum unit of an image.
A general TFT having a bottom gate structure includes, for example, a gate electrode provided on an insulating substrate, a gate insulating film provided so as to cover the gate electrode, an island-shaped semiconductor layer located on the gate insulating film so as to overlap the gate electrode, and a source electrode and a drain electrode facing each other on the semiconductor layer.
In the TFT having the bottom gate structure, an upper portion of a channel region is covered by an interlayer insulating film made of SiO2, etc., and a pixel electrode is provided on the interlayer insulating film to fabricate a thin film transistor substrate. A counter substrate is provided so as to face the thin film transistor substrate, and a liquid crystal layer is provided between the thin film transistor substrate and the counter substrate, thereby forming a liquid crystal display device.
In a conventional TFT structure, the semiconductor layer is located above the gate electrode, and therefore the gate electrode serves as a light shielding film. However, in the liquid crystal display device, a display region is irradiated with light of a backlight unit from the thin film transistor substrate side. Thus, once the light coming through a portion other than the light shielding film reflects on the counter substrate, etc., and comes into the semiconductor layer from above the TFT, the channel region of the semiconductor layer made of amorphous silicon is irradiated with the light due to lack of the light shielding film. As a result, because of the photoexcitation, a leakage current may be present when the TFT is off, or the amorphous silicon may be degraded by the light. This may result in a reduction in TFT properties and display quality of the liquid crystal display device.
Thin film transistor substrates configured to avoid such a disadvantage have been suggested. More specifically, in a thin film transistor substrate in which a TFT is located near an intersection between a gate line and a source line and in which the TFT and a pixel electrode are connected to each other, a metal layer for blocking light is provided above a channel region of the TFT via an insulating film (see, e.g., Patent Document 1).